Most of the non-contact displacement sensors available presently can only perform a single point measurement. The commonly used techniques include capacitive, inductive, magnetic and light reflection sensing. In the case of capacitive sensing, the electrical capacitance between a probe and a target is converted into a voltage output. The displacement between the probe and the target forms the dielectric thickness of the capacitor and hence changes in its thickness will change its electrical capacitance. The target is usually grounded for this type of displacement sensing in order to form a closed circuit. In magnetic displacement sensor, the magnetic field changes according to the air gap between the sensor head and the object.
Light reflection sensor is of the most commonly used sensor in which a light beam is transmitted and its reflection bouncing off the object is received and analysed. Displacement measurement is based on the interaction between the field of illumination of the transmitting fibers and the field of view of the receiving fibers. At contact, or zero gap, most of the light exiting the transmitting fibers is reflected directly back into those fibers. No light is provided to the receiving fibers and the output signal is "zero". As the probe to target displacement increase, increasing amounts of light are captured by the receiving fibers. This relationship will continue until the entire face of the receiving fiber is illuminated with reflected light. This point is called the "optical peak" and corresponds to the maximum voltage output. After the optical peak is reached, a continued increase in displacement will cause the diverging field of reflected light to exceed the field of view of the receiving fibers, producing a reversal in the output versus displacement signal relationship.
For greater standoff displacement measurement, the light sensing technique is not effective. A simple triangulation principle is used instead to calculate the displacement between the object and the transmitter. The light receiver is not a simple photodetector that converts the light level into voltage signal. Instead, a position sensitive device (PSD) is used to locate the center of the reflected light beam. The position of light beam is then used to calculate the displacement. The light source can be generated from a Light Emitting Diode (LED) or a Laser.
In order for a single point displacement sensor to obtain a complete surface profile, that is the 3D data of an object, the single point displacement sensor must be made to scan over the entire surface. The major disadvantage of performing scanning is the time taken is long. In addition, high precision mechanism are required for this type of setup.
There are optical methods that are capable of performing 3D measurement over an area without the need for point to point scanning. These methods are holographic interferometry, Moire techniques and speckle photography. The holographic method requires the fabrication of hologram for each and every object. Similarly, the Moire technique must begin with pre-fabricated diffraction gratings.